System for storing petroleum products

ABSTRACT

A system and process for storing petroleum products characterized by an underground manifold into which a plurality of underground tanks are vented and which in turn is vented to the atmosphere by a single line. A valving system is employed in conjunction with the vent lines running between the individual tanks and the manifold which automatically closes the vent line during the tank filling operation and stops the flow of the product at a level which provides sufficient additional tank storage capacity to subsequently accommodate the quantity of product in the fill hose. The valving system includes a venting bypass line which can be then opened to recycle the valving system, to permit the quantity of product in the fill line to flow into the tank and to vent the tank to the atmosphere through the manifold.

United States Patent [72] Inventor James P. Benson 54 Gibbon Blvd.,Cockeysville, Md. 21030 [211 App]. No. 695,263 [22] Filed Jan. 2, 1968[45] Patented Feb. 16, 1971 [54] SYSTEM FOR STORING PETROLEUM PRODUCTS15 Claims, 4 Drawing Figs.

[52] U.S. CI 137/202, 87/266 1.37/409; 220/86 [51] Int. Cl F16k 31/18[50] Field of Search 137/202, 266, 267, 213, 214, 401, 403, 409,404,190, 427, 583, 587, 388--390; 220/86 [56] References Cited 7 UNITEDSTATES PATENTS 334,481 l/l886 Sone 137/266X 668,008 2/1901 Cunningham137/266 1,844,297 2/1932 Roberts..... 220/86 2,948,294 8/1960 Smith137/266X 3,334,645 8/1967 Weinstein 3,458,085 7/1969 Clemenson PrimaryExaminer-William F. ODea. Assistant Examiner-Edward J. EarlsAttorney-Robert L. Berger ABSTRACT: A system and process for storingpetroleum products characterized by an underground manifold into which aplurality of underground tanks are vented and which in turn is vented tothe atmosphere by a single line. A valving system is employed inconjunction with the vent lines running PATENTEDFEBIGIB?! J 3563.263

" SHEET 10F 2 I N VE NTOR. JAMES P. BENSON ATTORNEY SYSTEM FOR STORINGPETROLEUM PRODUCTS BACKGROUND OF THE INVENTION 1. Field of the InventionThe invention relates to petroleum storing systems and, moreparticularly, to such systems wherein a plurality of storage tanks arelocated underground and vented to the atmosphere.

2. Description of the Prior Art conventionally, storage systems forpetroleum products at locations such as service stations involve the useof a plurality of underground storage tanks filled with products ofdifferent quality. A fill line is provided in each tank and the filllines and tanks are so positioned that the fill caps are in closeproximity to facilitate simultaneous loading of a plurality of the tanksfrom a single truck. The products from the tanks are pumped throughunderground lines to dispenser located on an "island, next to which thecustomer vehicles are parked.

Each tank is provided with a vent line that runs underground on a risingslope to connect. with a riser which extends a substantial distanceabove the ground. In this manner, the fumes from the product stored inthe tank are not expelled in a dangerous location or where they wouldlikely be offensive to persons in the vicinity of the station.Typically, codes require that risers in an open area extend at least 12feet above the finished grade at its base. Alternately, they arenormally required to extend 3 feet above the roof of a building and tobe horizontally positioned 5 feet from any opening in the buildmg.

In any event, each tank has its own venting system. A fivetank stationhas heretofore required the installation of five separate ventingsystems typically employing 2 inch piping and running a distance ofanywhere from 25-feet to as much as 100 feet, with the major portionthereof being installed below grade level. Multiple venting systems havebeen found necessary in the past to prevent the flow of product from onetank to another. For instance, during filling operations it is notunusual for the operator to load a tank beyond its capacity. Under suchcircumstances, if the plurality of tanks were vented by aninterconnected venting system, the product in the tank being filledcould flow through the vent lines into other tanks, therebycontaminating the product contained therein.

Quite obviously, the prior art practice of providing each tank with itsown venting system has significantly increased storage systeminstallation costs. This is a major factor to be considered in anyhighly competitive field, particularly in these times of rapidly risingcosts. Additionally, multiple risers detract from the aesthetic qualityof the environs.

Another characteristic of these prior art storage systems is that, in anoverloading situation, product is pushed into the vent line. Frequently,the slope of the underground portion of the vent line is not true andsome of the product will remain in the line as the tank is emptied. Thenext time a delivery is made, vapors from the tank are expelled withforce through the venting system causing the product trapped therein tospray out of the vent riser. This not only presents an unsanitarycondition but also one which is potentially dangerous.

An analogous situation of far more serious consequences frequentlyarises during the tank filling operation. These tanks are filled from atank truck utilizing a fill hose provided with a visual gauge. Theoperator watches the gauge and, as the flow rate in the hose decreases,he knows that the tank is almost full. At this time, he should stop theflow in order that the tank will still accommodate the product,typically on the order of to gallons, remaining in the fill hose.However, if for one reason or another, the operator does not stop theflow at the proper time, he finds himself with a full tank and hose. Ifthe delivery is being made when the station is in operation, the tanktruck operator can wait until the station operator has sold enoughproduct to drain the hose. Altemately, the product can be dispersedthrough the service station pump into buckets and dumped into anothertank. This is not a safe operation. Neither is completely ethical whenthe other tank contains a product of different quality. Obviously, theproblem is compounded if such a situation arises at a time when thestation is closed. In this instance, a spill is inevitable and a verydangerous and otherwise undesirable situation is created.

These limitations of prior art systems for storing petroleum productsare alleviated by the present invention.

It is therefore a primary object of this invention to provide a storagesystem for petroleum products which is significantly less expensive thansystems currently in use.

i It is another object of this invention to provide a common ventingsystem for a plurality of underground tanks.

A further object is to prevent petroleum products in a storage tank fromentering the tank's venting system.

Another object of this invention is to automatically stop the flow ofproduct into a storage tank at a level which will permit product in thefill hose to subsequently be accommodated in that the tank.

An additional object is to prevent :spilling of product in servicestation areas during storage tank filling operations.

Also, one object of this invention is to provide a unique and improvedvalve which prevents petroleum storage systems from being filled beyonda predetermined volumetric capacity.

A still further object of this invention is to provide a manifoldedventing system for a plurality of petroleum product storage tanks, whichwill preclude the passage of product from one tank into another underany possible conditions.

BRIEF DESCRIPTIONOF THE DRAWINGS The novel features that are consideredcharacteristic of the invention are set forth with particularity in theappended claims. The invention itself, however, both as to itsorganization and its method of operation, together with additionalobjects and advantages thereof, will best be understood from thefollowing description of specific embodiments when read in connectionwith the accompanying drawings, wherein like numbers have been employedin the different figures to denote the same parts and wherein:

FIG. 1 is a perspective view illustrating the improved system of thisinvention for storing petroleum products;

FIG. 2 is a detail elevation view in section of the preferred embodimentof the unique valve employed in the storage system of FIG. 1;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2; and

FIG. 4 is a perspective view of the manifold of the storage system ofFIG. I and the connections thereto.

SUMMARY OF INVENTION The invention comprises both a system and processfor storing petroleum products. Briefly, each of a plurality ofunderground storage tanks is vented to a common manifold located intheir immediate vicinity. A single vent line runs from the manifold to aconvenient, above grade, relatively distant location where the vaporscan be safely exhausted. A float valve connects each tank to its ventline and is designed to close off the vent line prior to the tank'sreaching its full capacity. The flow of product into the tank will ceaseas soon as the vapor pressure equals the head on the product in the fillhose. Holes in the float permit the float to be gradually flooded,thereby losing its buoyancy. A secondary venting system is manuallyopened, bypassing the float valve. At this point, the float valverecycles to its original open position and the product remaining in thefill hose will flow into the tank. Product in the float drains throughone of the holes provided therein as product is emptied from the storagetank to restore the float's buoyancy.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, threepetroleum product storage tanks 10, 11 and 12 are shown buried under thepaved surface 13 of a service station. These tanks 10, 11 and 12 arerespectively provided with fill pipes 14, 15 and 16 which extend fromthe paved surface 13 through the top of the tank to the vicinity of thebottom of the tank, and with suction lines 17, 18 and 19, which extendunderground to a service station island 20 where they connect todispensing pumps 21, 22 and 23. Typically, different grade product isstored in each of the tanks 10, 11 and 12 and vehicles being filled withpetroleum park next to island 20 and are serviced in the conventionalmanner through the dispensing pumps 21, 22 and 23. A manifold 24 isinstalled under the paved surface 13 above the tanks 10, 11 and 12 andin the immediate vicinity thereof. Mounted to the top of each tank 10,11 and 12 is a unique valve 25 of this invention, the details andoperation of which will subsequently be fill fully explained. Themainfold 24 is connected to the valves 25 of the tanks 10, 11 and 12 bypipes 26, 27 and 28, respectively, which are installed on a graduallyrising slope between the valve and the manifold. Also connected to themanifold 24 is a vent line 29 which runs a relatively long distanceunderground on a gradually rising slope from the manifold to a pointadjacent a wall 30 of the service station building 31 where it turnsvertically upwards to form a riser 32 positioned adjacent the wall andrising above the roof 33.

It will be noted that the fill pipes 14, 15 and 16 are positioned inclose proximity. In this manner, they can be filled from a tank truck 34part parked in one position in the station. Due to the number of taskswhich must be performed by the truck operator, it has become thepractice to fill no more than two tanks simultaneously from the truck34. Two fill hoses 35 are seen connected to tanks 10 and 11 and toseparate tank compartments (not illustrated) on the truck 34 throughunloading valves 36. Tank 12 is subsequently loaded in a similar mannerthrough fill pipe 16 and one of the hoses 35 after delivery is completedto either tank 10 or 11.

The detail structure and operation of the unique valve 25 will be bestappreciated by referring to F168. 2 and 3. A valve body 37 is providedwith a cylindrical bore 33 which extends therethrough with one endprovided with internal threads 39 to facilitate connection to one of thepipes 26, 27 or 28. The other end of the cylindrical bore 38 is providedwith internal threads 40 to receive the threaded collar 41 whichterminates in an outwardly depending flange 42. A float 43 is pivotallyconnected to the valve body 37 by an arm 44 which carried a disc 45mounted thereto on a swivel joint 46. As illustrated, in the fullyextended position 47 of the float 43, the disc 34 is not in contact withthe annular cap 41; however, when in its uppermost position 48, the discseats against the flange 42 on the collar 41 to close off the entrance49 to the valve body 37. Any initial misalignment between the flange 42of collar 41 and disc 45 is accommodated by the swivel joint 46, therebyassuring a gas tight seal between those two members. Collar 41 and disc45 are formed of closely machined metal parts or of other materialssuitable to effect a gas tight seal therebetween, such as neoprene orpolyurethane, when the float 43 is in its uppermost position 48. Smallapertures 50 and 51 are provided through the upper and lower portions,respectively, of float 43. Their function will subsequently becomeapparent.

Also provided in the valve body 37 is a second cylindrical bore 52, thelower end of which communicates with the bore 38 through a channel 53 inthe valve body. This bore 52 intersects a tubular member 54,communicating with the bore 38, and which tubular member is providedwith an end cap 55 having an aperture 56. Positioned within the bore 52is a piston 57 which has a tapered groove 53 and which is connected to acable 59 aligned axially with the bore and extending externally thereofthrough a cap 60 mounted on a threaded extension 61 of the bore formedon the valve body 37. That portion of the cable 59 extending exteriorlyof the valve body 37 is provided with a shield 90. A spring 62 withinthe bore 52 acts against the piston 57 to continuously bias it in thedirection of the channel 53.

Mounted within the tubular member 54 is a bearing support structure 63which slideably carries a pin 64, one end of which carries an annularseal 65 and the other end 66 of which engages the tapered groove 58. Thebearing support structure includes apertures 67 spaced around itsperiphery and an annular seat 68. A spring 69 mounted within the bearingsupport structure 63 between the end cap 55 and the annular seal 65urges the annular seal and the pin 64 towards the piston 57. Action ofthis spring 69 causes the seal 65 to seat against the annular seat 68when the piston 57 is fully extended within the bore 52. The annularseal 65 may for formed of neoprene or any other suitable material whichwill effect a gastight connection with the seat 68.

Pulling the cable 59 in an upward direction will cause the taperedsurface of the groove 58 to drive the pin 64 against the force of thespring 69, thereby disrupting the gastight connection between the seal65 and the seat 68. Packing material 70 within the threaded extension 61acts against the cable 59 to maintain a gastight seal as the cable ismoved axially within the bore 52. As illustrated in the drawing, thecylindrical bore 52 is parallel with the bore 38. However, in manyinstances, it will be desirable to locate the upper ends of the shieldedcables 59 in the same fill boxes wherein the fill pipes 14, 15 and 16terminate. Under such circumstances, easiest operation of the shieldedcables 59 will be obtained when the axes of the bores are aligned withtheir respective fill boxes.

The valve body 37 is connected to the storage tank l0, l1 and 12utilizing screw threads 71 provided around its periphery. These threadscooperate with threaded ring 72 which is positioned within an opening 73in the top of the tank 10, 11 or 12 and which is welded to the tank,

Referring now to FIG. 4, the manifold 24 is seen to comprise three teepipe fittings 74, 75 and 76 joined together by short nipples 77 and 78.An elbow 79 connects the tee 74 to pipe 26. Similarly, elbows 80 and 81connect tees 75 and 76 to pipes 27 and 28, respectively. Closing one endof the manifold 24 is a plug 82. The other end of the manifold 24 isconnected to the vent line 29 by two elbows 83 and 84 joined by a shortnipple 85. Typically, the pipes 26, 27 and 28 are 2 inches in diameter,the vent line 29 is 3 inches in diameter and the manifold 24 is formedof 3 inches X 3 inches X 2 inches tees 74, 75 and 76.

OPERATION OF THE PREFERRED EMBODIMENT In order to fill the storage tanks10, 11 and 12, the tank truck 34 is first parked in the vicinity of thefill pipes 14, 15 and 16. Fill hoses 35 are connected between theunloading valves 36 and the fill pipes 14 and 15. The respectiveunloading valve 36 are then opened to permit products in the tank truck34 to simultaneously flow through fill hoses 35 and the fill pipes 14and 15 into the tanks 10 and 11. As product flows into each tank, suchas tank 10, it will gradually fill the tank and initially come intocontact with float 43 in its position 47. This float 43 will then risewith the rising product level within the tank 10 until it reaches itsuppermost position 48. Until the float 43 reaches this uppermostposition 48, the rising product level in the tank 10 forces vapors inthe tank to be exhausted to the atmosphere through the cylindrical bore38 of the valve body 37, the pipe 26, the manifold 24 and the vent line29. When the float 43 is in its uppermost position 48, the disc 45 seatsagainst the flange 42 of the collar 41 to preclude further escapeofvapors from within the tank 10 through the entrance 49 of the valvebody 37 and subsequently through the vent line 29 to the atmosphere.After the disc 45 has seated against the flange 42, a small additionalamount of product will flow from the truck 34 into the tank 10 until thepressure of the vapor retained in the tank above the product equals thehead pressure of the product within the fill hose 35 of the truck 34.However, when this condition is reached, flow of product into the tankautomatically ceases. It will be noted that the tank is not filled tocapacity at this time and the product level will be well below theaperture 56.

During this phase of the operation, an the annular seal 65 within thetubular member 54 remains in contact with the seat 68 under pressurefrom spring 69. Thus, none of the vapors in the tank escape into theventing system through the opening 56 and the tubular member 54.

Also during this phase of the operation an after the rising productwithin the tank 10 has initially contacted the float 43, product verygradually, but continuously, flows into the float through the smallaperture 51. As product flows into the float 43 through aperture 51,vapors contained therein escape through aperture 50. These apertures 50and 51 are sized so that the float 43 does not lose its buoyancy untilafter the float is in its uppermost position 48 with the disc 45 firmlyseated against the flange 42 of the collar 41. After the disc 45 isseated against the flange 42, product continues to flow into the float43 through aperture 51 until the level of the fluid within the float isat the same level as that of the product surrounding the float. Thefloat 43 is configured so that when this condition is reached the floatloses its buoyancy. i

As indicated, shortly after the disc 45 seats against the flange 42 topreclude further escape of vapors from the tank 10 into the ventingsystem, flow of product in the fill hose aus tomatically ceases and thefloat 43 loses its buoyancy. However, since the pressure on the bottomof the disc 45 is greater than the atmospheric pressure against its top,the disc and attached float 43 will not return to their original fullyextended annular seat 68. Vapors within the tank 10 then escape throughaperture 56, the bearing support structure 63, the apertures 67, thetubular member 54 and the cylindrical bore 38 into the venting system.Product within the fill hose 35 will then drain into the tank 10 and,when the vapor pressure within the tank reaches atmospheric, the float43 returns to its original position 47 with the interior of the tankvented through the entrance 49 of the valve body 37.

The truck operator releases the cable 59 and the springs 62 and 69 willreturn the piston 57 and the annular seal to their original positions,respectively. As indicated, this operation is concurrently performedwith respect to tank 11. As soon as the filling operation with respectto the first of these tanks 10 and 11 is completed, its fill hose 35 isconnected between the fill pipe 16 of tank 12 and a selected tankcompartment (not illustrated) on the truck 34 through a valve 36. Theoperation is then repeated with respect to tank 12 to fill it withproduct. After all tanks 10, 11 and 12 have been filled, the operatorcan disconnect his fill hose 35 and make his next delivery.

The valve 25 is configured so that the product within its respectivetank 10, 11 or 12 never rises to the level of aperture 56. in manyinstances, it may be desirable to never let the level of the productreach the entrance 49 to the venting system.

As product is dispensed through, for instance, the 5 dispensing pump 21to a point where the level of the product in the tank 10 is below thefloat 43, all of the product in the float will have drained out throughaperture 51, thereby restoring the float to its full buoyancy condition.At this point,

the venting system is completely recycled and ready for the 70 next tankfilling operation, i.e., the float is fully buoyant and in it fullyextended position 47, the tank 10 is connected to the venting systemthrough the entrance 49 of the valve body 37 and the seal 65 is incontact with the annular seat 68. For one reason or another, deliveriesare often made which only partially fill tanks 10, 11 or 12. Naturally,in these instances, the vapors will be vented through the entrance 49 ofthe valve body 37 to atmosphere, but operation of the valve 25 willotherwise not be affected.

Channel 53 facilitates the removal of any foreign matter which maycollect in the cylindrical bore 52.

It will be appreciated by those familiar with this art that this systemprecludes, under all condition, product from entering the ventingsystem. One of the more significant advantages of this aspect of theinvention is the substantial cost savings which can be realizedutilizing an underground manifold connected to each of a plurality oftanks and which is vented to the atmosphere by a single line. Thissystem also advantageously automatically cuts off the flow of productfrom the tank truck while there is still sufficient capacity in thetanks to accommodate the amount of product in the fill hoses.

I claim: 1. A venting system for a plurality of underground liquidstorage tanks each having fill and outlet connections comprismg:

an underground manifold; a pipe connecting each said tank to saidmanifold; a single vent line having one end thereof connected to saidmanifold and its other end opening to the atmosphere;

first means carried by each said tank for automatically closing off itsrespective said pipe when said fluid in said tank reaches apredetermined level below the full capacity of said tank; and

second means for subsequently opening said respective pipe while saidfluid in said tank is at least at said predetermined level.

2. The system of claim 1 wherein said first means includes:

a valve body including communicating open ends interposed between saidtank and its respective said pipe; and

a float moveably attached to said valve body for closing off one saidopen end of said valve body when said float is in its uppermostposition.

3. The systemof claim 2 wherein said second means includes means forremoving the buoyancy of said float.

4. The system of claim 3 wherein said second means additionally includesmeans connected to said valve body for venting the interior of said tankto said respective pipe independently of said first means.

5. The system of claim 2 wherein said second means includes means forrestoring said float to a valve-open position with the interior of saidtank communicating with said respective pipe. I

6. A valve assembly for use in venting a fluid storage tank having afill pipe and a vent line comprising:

a valve body connected between said vent line and the upper interior ofsaid tank;

first means connected to said valve body responsive to the level offluid within said tank for automatically closing said valve when saidfluid read reaches a predetermined level within said tank below the fullcapacity of said tank; and

second means for subsequently restoring said first means to a valve-openposition while said fluid is at least at said predetermined level.

7. The valve assembly of claim 6 wherein said first means comprises:

a float moveably attached to said valve body; and

closure means carried by said float to seat against the bottom of saidvalve body and thereby close said valve when said fluid reaches saidpredetermined level in said tank.

8. The valve assembly of claim 7 wherein said second means includesmeans for removing the buoyancy of said float.

9. The valve assembly of claim 8 wherein said second means additionallyincludes means connected to said valve body for venting the interior ofsaid tank to said vent line independently of said first means.

10. The valve assembly of claim 8 wherein said means for removing thebuoyancy of said float includes at least two apertures in said float.

11. A system for venting vapor from a plurality of underground liquidstorage tanks each having liquid fill and liquid outlet connectionscomprising:

an underground manifold;

a single vent line having one end thereof connected to said manifold andthe other end thereof opening to the atmosphere;

a plurality of vent pipes corresponding in one-to-one relation to thestorage tanks with a separate one of said vent pipes connecting theinterior of its corresponding storage tank to said manifold;

first means acting in conjunction with each said vent pipe and itscorresponding tank for sharply decreasing the rate at which vapor can bevented from the interior of each tank to its corresponding vent pipewhen liquid in the storage tank reaches a predetermined level; and

second means for subsequently increasing the rate at which vapor can beso vented while the liquid in the storage tank is at least at itspredetermined level.

12. The system of claim 11 in which the means for sharply decreasing therate at which vapor may be vented from the interior of each tank to itscorresponding vent pipe includes float actuated means for blocking atleast the largest portion of the vapor flow from the interior of thestorage tank to its corresponding vent pipe when liquid in the tankreaches a predetermined level.

13. The system of claim 12 in which said second means comprises meansfor removing the buoyancy of said float actuated means.

14. The system of claim 11 in which said means for sharply decreasingthe rate at which vapor can be vented from a tank to its correspondingvent pipe includes:

first and second orifices connecting the interior of the tank with itscorresponding vent pipe, said first orifice being substantially largerthan said second orifice; and

float actuated means for closing said first orifice when liquid in thetank reaches a predetermined level.

15. The system of claim 14 in which said second means comprises meansfor removing the buoyancy of said float actuated means.

1. A venting system for a plurality of underground liquid storage tankseach having fill and outlet connections comprising: an undergroundmanifold; a pipe connecting each said tank to said manifold; a singlevent line having one end thereof connected to said manifold and itsother end opening to the atmosphere; first means carried by each saidtank for automatically closing off its respective said pipe when saidfluid in said tank reaches a predetermined level below the full capacityof said tank; and second means for subsequently opening said respectivepipe while said fluid in said tank is at least at said predeterminedlevel.
 2. The system of claim 1 wherein said first means includes: avalve body including communIcating open ends interposed between saidtank and its respective said pipe; and a float moveably attached to saidvalve body for closing off one said open end of said valve body whensaid float is in its uppermost position.
 3. The system of claim 2wherein said second means includes means for removing the buoyancy ofsaid float.
 4. The system of claim 3 wherein said second meansadditionally includes means connected to said valve body for venting theinterior of said tank to said respective pipe independently of saidfirst means.
 5. The system of claim 2 wherein said second means includesmeans for restoring said float to a valve-open position with theinterior of said tank communicating with said respective pipe.
 6. Avalve assembly for use in venting a fluid storage tank having a fillpipe and a vent line comprising: a valve body connected between saidvent line and the upper interior of said tank; first means connected tosaid valve body responsive to the level of fluid within said tank forautomatically closing said valve when said fluid read reaches apredetermined level within said tank below the full capacity of saidtank; and second means for subsequently restoring said first means to avalve-open position while said fluid is at least at said predeterminedlevel.
 7. The valve assembly of claim 6 wherein said first meanscomprises: a float moveably attached to said valve body; and closuremeans carried by said float to seat against the bottom of said valvebody and thereby close said valve when said fluid reaches saidpredetermined level in said tank.
 8. The valve assembly of claim 7wherein said second means includes means for removing the buoyancy ofsaid float.
 9. The valve assembly of claim 8 wherein said second meansadditionally includes means connected to said valve body for venting theinterior of said tank to said vent line independently of said firstmeans.
 10. The valve assembly of claim 8 wherein said means for removingthe buoyancy of said float includes at least two apertures in saidfloat.
 11. A system for venting vapor from a plurality of undergroundliquid storage tanks each having liquid fill and liquid outletconnections comprising: an underground manifold; a single vent linehaving one end thereof connected to said manifold and the other endthereof opening to the atmosphere; a plurality of vent pipescorresponding in one-to-one relation to the storage tanks with aseparate one of said vent pipes connecting the interior of itscorresponding storage tank to said manifold; first means acting inconjunction with each said vent pipe and its corresponding tank forsharply decreasing the rate at which vapor can be vented from theinterior of each tank to its corresponding vent pipe when liquid in thestorage tank reaches a predetermined level; and second means forsubsequently increasing the rate at which vapor can be so vented whilethe liquid in the storage tank is at least at its predetermined level.12. The system of claim 11 in which the means for sharply decreasing therate at which vapor may be vented from the interior of each tank to itscorresponding vent pipe includes float actuated means for blocking atleast the largest portion of the vapor flow from the interior of thestorage tank to its corresponding vent pipe when liquid in the tankreaches a predetermined level.
 13. The system of claim 12 in which saidsecond means comprises means for removing the buoyancy of said floatactuated means.
 14. The system of claim 11 in which said means forsharply decreasing the rate at which vapor can be vented from a tank toits corresponding vent pipe includes: first and second orificesconnecting the interior of the tank with its corresponding vent pipe,said first orifice being substantially larger than said second orifice;and float actuated means for closing said first orifice when liquid inthe tank reaches a predetermined level.
 15. The system of claim 14 inwhich said second means comprises means for removing the buoyancy ofsaid float actuated means.